A probe for removing a gas sample from a hot reaction space is known from DE 103 15 996 A1, wherein cooling water flows through an annular space bounded by an outer casing. Since the boiling point of the water must not be reached, the maximum return temperature is around 90° C. However, at these low temperatures, it is not possible to reliably rule out situations in which the temperature drops below the dew point within the gas sampling probe. Therefore, some manufacturers additionally heat the gas sampling tube electrically, in order in this way to prevent situations in which the temperature drops below the dew point in the measurement gas. In DE 103 15 996 A1, the temperature is prevented from dropping below the dew point in that the gas sampling tube is surrounded by an evacuable cavity. However, strong water cooling also has the further drawback that the outer part of the gas sampling probe is cooled to an unnecessarily great extent, wherein an excessively cold tip of the gas sampling probe can tend toward the formation of deposits in the hot process gas.
Also known are oil-cooled gas sampling probes which use a heat transfer oil for cooling. The difference from the water circuit is that the heat transfer oil can be used in higher temperature ranges. As a result, it is possible to dispense with additional heating of the gas sampling tube. A problem, however, is that it is difficult to assess the heat transfer oil with regard to leaks, since it is not possible to rule out a fire hazard here.
Furthermore, a high-temperature sampling probe is known from DE 103 54 188 A1, wherein a gas sampling tube is enclosed by an outer tube and pressurized cooling air is conducted between these two tubes. In the process, care was taken to ensure that the removed gas to be analyzed is not cooled to below 250° along the gas sampling tube and so condensation is avoided.
Accordingly there is a need for a method of operating a gas sampling probe, by which sufficient cooling of the front end of the gas sampling tube is ensured but reliably avoids dropping the temperature of the gas to be analyzed and the components thereof below their dew point temperature.